This invention relates to a force transmitting mechanical gear molded from a synthetic resin, and more particularly to a gear structure used, for example, in a vehicle starter having a reduction gear mechanism.
A reduction gear type vehicle starter is well known, as shown in Japanese Utility Model publications Nos. 67977/83, 103567/83, 104361/83, 120874/83, 134056/83, and 134676/83, and in U.S. Pat. No. 4,346,615.
FIGS. 1 and 2 show a conventional vehicle starter having a gear reduction mechanism, wherein an internal gear 1a is formed on of the inner surface of a stationary intermediate bracket 1 made of synthetic resin. A sleeve bearing 2 is firmly fixed to a flange portion of the bracket. A cylindrical yoke 4 is coupled to a faucet joint portion 3a of a front bracket 3 overlying the bracket 1. A plurality of ferrite magnetic poles 5 are fixed to the inner surface of the yoke 4. An armature core 6, an armature coil 7 and an armature shaft 8 are rotatably disposed in the space formed by the cylindrical yoke.
A direct current motor M is composed of the yoke 4, poles 5, armature core 6, armature coil 7 and armature shaft 8. A packing 13 is disposed between the adjacent end portions of the yoke 4 and the intermediate bracket 1. An external sun gear 8a is formed on part of the outer surface of the armature shaft 8. A planet gear 9, having a sleeve bearing 9a fitted into its inner surface, meshes with the internal gear 1a and the external gear 8a, and is driven by the external gear. A pin 10 is fixed to a flange portion 11a of an output shaft 11. A ball bearing 12 for rotatably supporting the output shaft 8 is disposed between the armature shaft 8 and the output shaft 11.
With such a starter construction, when the motor M is energized the armature shaft 8 is rotated, and the planet gear 9 is driven by the external gear 8a formed on the shaft 8. The planet gear thus rides around the internal gear 1a, and transmits a rotating force with decreased speed from the armature shaft 8 through pin 10 and flange portion 11a to the output shaft 11.
In such a conventional reduction gear starter, the internal gear is made of molded synthetic resin such as nylon resin containing only one filler, such as carbon fibers or glass fibers. A gear made according to this conventional method has certain defects, however, as described below. When glass fibers are mixed into the synthetic resin mold material, although shock resistance improves, wear resistance deteriorates. On the other hand, when carbon fibers are mixed into the synthetic resin molding material, although wear resistance improves, shock resistance deteriorates. The cost of a gear made with carbon fibers is higher than that of a gear made with glass fibers as described above. In each case, satisfactory mechanical characteristics, such as flexible strength, shock resistance, required coefficient of friction, and wear resistance, cannot be obtained.